The invention relates to a method for determining the unambiguous range for the measurement of the intermediate frequency deviation in a frequency-regulated radar system.
In frequency pulse radar systems, in which the echo signal is transformed or translated by means of frequency shifting to an intermediate frequency (IF) (as disclosed in German Patent DE 41 04 907 C2 and corresponding published European Patent Application 0,499,706), the frequency spacing between the transmitting- and LO-signal must be constant, so that the IF-signal falls into the IF band. A change of the frequency spacing in free-running oscillators due to external conditions (for example temperature or aging) leads to limitations in the system sensitivity, which can even lead to signal failures or losses.
In many radar systems with free-running oscillators, a frequency regulation can be utilized, with which the frequency deviation of the IF-signal can be measured using the available evaluating algorithms (as disclosed in German Patent DE 195 12 904 C2 and corresponding U.S. Pat. No. 5,864,313). In this method, the effect is used to advantage, that the deviation of the IF-signal from the nominal or rated reference frequency can also be determined in the video signal, and this deviation is large relative to arising Doppler frequencies. A prerequisite for the utilization of the method is the existence of an echo signal, that is to say of a target. After the determination of the IF-deviation, the transmitting- or LO-frequency can be regulated in a corresponding, i.e. tracking or following manner, until the deviation becomes zero.
A block circuit diagram of the method, as it is used, for example, in the radar sensor of a spacing distance regulating automatic speed control for a motor vehicle, is shown in FIG. 1. In the I/Q demodulator, the echo signal (IF-plane) is mixed onto the video plane and thereafter sampled. For the determination of the IF-deviation (special mode in the evaluating algorithm of the radar system) the sampling frequency fS is adjustingly set higher than in the signal processing for the detection of targets. From the phase difference of the sampled signals, the frequency shifting of the IF-signal may be determined and a corresponding value for the frequency detuning (tuning voltage UA) can be input into the front end.
The frequency range that can be unambiguously measured with the above described method is dependent on the sampling frequency fS. The relationship between the actual IF-deviation xcex94IF and the IF-deviation xcex94IFMeas determined with a sampling frequency fS is shown in FIG. 2. Within the interval of xc2x1fS/2, the IF-deviation can be unambiguously determined (unambiguous range), in the other ranges, ambiguities arise due to the under-sampling (ambiguous ranges).
For example, if the frequency measurement is carried out with a sampling frequency of 60 MHz, an unambiguous range of xc2x130 MHz can be achieved.
Under consideration of the mechanisms that typically lead to frequency detuning, in typical radar systems it can be assumed that the time constants of the frequency detuning are very large. Thus, in the typical operation of the radar system with an adequately frequent measurement of the frequency offset and sufficiently rapid regulation to the nominal or rated frequency, the unambiguous range of the frequency measurement will not be outside of xc2x1fS/2. When switching on the system, however, it cannot be assumed, that the IF-deviation lies in the unambiguous range of the measurement.
It is thus the object of the invention to improve the method for determining the IF-deviation according to the above discussed prior art in such a manner so that the unambiguous range can be identified. By corresponding frequency adjustment or tuning it can then be ensured that the measurement of the IF-deviation is achieved in the unambiguous range.
The above object has been achieved according to the invention in a method of determining an unambiguous range for a measurement of an IF-deviation in a frequency-regulated radar system, comprising receiving echo signals and carrying out a frequency adjustment of the radar system to respective adjustment settings during the receiving of the echo signals by setting at least one respective value around zero for the IF-deviation respectively in a first range and a second range of the IF-deviation, and recognizing the unambiguous range among the first range and the second range by comparing at least one of a signal amplitude and a number of targets that are detected respectively at the respective adjustment settings. In a first embodiment, the carrying out of the frequency adjustment comprises running in frequency steps through a certain frequency range that at least partially covers the unambiguous range and an ambiguous range. In a second embodiment, the carrying out of the frequency adjustment comprises: pre-specifying a fixed frequency starting value; measuring the IF-deviation at the fixed frequency starting value; carrying out a balancing adjustment to adjust to zero the IF-deviation that was measured; mixing the echo signals into a video plane and then sampling the echo signals at a sampling frequency; and carrying out an adjusted setting of a frequency offset relative to the fixed frequency starting value, wherein the frequency offset essentially corresponds to the sampling frequency.
The above objects have further been achieved according to the invention in a method of operating a frequency-regulated radar system comprising the following steps:
a) emitting a transmitted signal at a transmission frequency, and reflecting said transmitted signal from at least one target object to form a reflected echo signal including at least one echo signal pulse corresponding to said at least one target object;
b) receiving said echo signal, and mixing said echo signal with a reference frequency to form an intermediate frequency;
c) determining an IF-deviation of said intermediate frequency relative to said reference frequency;
d) adjusting at least one of said transmission frequency and said reference frequency;
e) carrying out said steps c) and d) to an extent to minimize said IF-deviation to near or equal to zero at a first tuned frequency in a first frequency tuning range of said at least one of said transmission frequency and said reference frequency;
f) detecting at least one of a first signal amplitude of said echo signal and a first number of said echo signal pulses of said echo signal received when said at least one of said transmission frequency and said reference frequency is adjusted to said first tuned frequency;
g) repeating said steps c) and d) to minimize said IF-deviation to near or equal to zero at a second tuned frequency in a second frequency tuning range of said at least one of said transmission frequency and said reference frequency, wherein said second frequency tuning range is distinct from said first frequency tuning range;
h) detecting at lest one of a second signal amplitude of said echo signal and a second number of said echo signal pulses of said echo signal received when said at least one of said transmission frequency and said reference frequency is adjusted to said second tuned frequency;
i) recognizing said first frequency tuning range as an unambiguous range of said IF-deviation and said second frequency tuning range as an ambiguous range of said IF-deviation if said first signal amplitude is greater than said second signal amplitude or said first number of said echo signal pulses is greater than said second number of said echo signal pulses, or recognizing said second frequency tuning range as an unambiguous range of said IF-deviation and said first frequency tuning range as an ambiguous range of said IF-deviation if said second signal amplitude is greater than said first signal amplitude or said second number of said echo signal pulses is greater than said first number of said echo signal pulses; and
j) adjusting said at least one of said transmission frequency and said reference frequency into said unambiguous range, and then operating said radar system in said unambiguous range.